Nuclear quadrupole resonance (NQR) is a phenomenon where certain atomic nuclei generate resonant signals when an oscillating magnetic field at a particular frequency is applied to the nuclei. Some atomic nuclei can generate resonant signals responsive to two or more different applied frequencies. The NQR resonant signals can be detected without an externally applied magnetic field. Different atomic nuclei will have different resonant frequencies. For example, the resonant frequencies of nitrogen are different from the resonant frequencies of chlorine. Also, atomic nuclei of the same chemical element that are located within different chemical species can have different resonant frequencies. For example, the nitrogen nuclei located within the ammonium nitrate will have different resonant frequencies from nitrogen nuclei located within RDX. Furthermore, atomic nuclei of the same chemical element that are located within different sites of a chemical species can also have different resonant frequencies. Such NQR phenomena can be used to determine properties of a substance.